Resolve issue where config from API is cast to incorrect model type (faulty discriminator logic)

With our library of 300+ sources and 60+ destinations, certain API endpoints should return a "configuration" that is typed to the correct source or destination class, but they don't properly deserialize into the proper classes. Instead, they attempt to deserialize into the first match alphabetically (e.g. "Airtable" instead of "Snowflake" or "MySQL").

I've worked around this by hacking a bit and getting the original raw dict object, but this has been a stumbling block for specific use cases.

Workaround logic is here:

https://github.com/airbytehq/PyAirbyte/blob/f7b88eba400d7aa768c8c370cfbac6f18dfc61c6/airbyte/_util/api_util.py#L577-L597

----

Speakeasy has some docs on how to set up discriminator logic here:

- https://www.speakeasy.com/openapi/schemas/objects/polymorphism#discriminator-object-in-openapi

Example code from the docs:

```yaml
components:
responses:
OrderResponse:
oneOf:
- $ref: "#/components/schemas/DrinkOrder"
- $ref: "#/components/schemas/IngredientOrder"
discriminator:
propertyName: orderType
```

In our case, the descriminating property does exist in the data as `sourceType` and `destinationType`, but it is not defined with the above syntax.

Here is an example declaration which shows `sourceType` should be ready to leverage if we reference it in the `descriminator` declaration:

```yml
source-aha:
type: "object"
required:
- "api_key"
- "url"
- "sourceType"
properties:
api_key:
type: "string"
title: "API Bearer Token"
airbyte_secret: true
description: "API Key"
order: 0
x-speakeasy-param-sensitive: true
url:
type: "string"
description: "URL"
title: "Aha Url Instance"
order: 1
sourceType:
title: "aha"
const: "aha"
enum:
- "aha"
order: 0
type: "string"
```

-----

Current it does not appear that we define any descriminator logic to [`DestinationConfiguration`](https://arc.net/l/quote/xgczbtsc) or [`SourceConfiguration`](https://arc.net/l/quote/inshxepe).

Below is destination configuration. Note there is `oneOf` logic but no discriminator logic defined. Same for `SourceConfiguration`, although I'm not showing it because it is much larger.

Show/Hide

https://raw.githubusercontent.com/airbytehq/airbyte-platform/refs/heads/main/airbyte-api/server-api/src/main/openapi/api_sdk.yaml

```yml
DestinationConfiguration:
description: The values required to configure the destination.
example: { user: "charles" }
oneOf:
- title: destination-google-sheets
$ref: "#/components/schemas/destination-google-sheets"
- title: destination-astra
$ref: "#/components/schemas/destination-astra"
- title: destination-aws-datalake
$ref: "#/components/schemas/destination-aws-datalake"
- title: destination-azure-blob-storage
$ref: "#/components/schemas/destination-azure-blob-storage"
- title: destination-bigquery
$ref: "#/components/schemas/destination-bigquery"
- title: destination-clickhouse
$ref: "#/components/schemas/destination-clickhouse"
- title: destination-convex
$ref: "#/components/schemas/destination-convex"
- title: destination-databricks
$ref: "#/components/schemas/destination-databricks"
- title: destination-dev-null
$ref: "#/components/schemas/destination-dev-null"
- title: destination-duckdb
$ref: "#/components/schemas/destination-duckdb"
- title: destination-dynamodb
$ref: "#/components/schemas/destination-dynamodb"
- title: destination-elasticsearch
$ref: "#/components/schemas/destination-elasticsearch"
- title: destination-firebolt
$ref: "#/components/schemas/destination-firebolt"
- title: destination-firestore
$ref: "#/components/schemas/destination-firestore"
- title: destination-gcs
$ref: "#/components/schemas/destination-gcs"
- title: destination-iceberg
$ref: "#/components/schemas/destination-iceberg"
- title: destination-milvus
$ref: "#/components/schemas/destination-milvus"
- title: destination-mongodb
$ref: "#/components/schemas/destination-mongodb"
- title: destination-motherduck
$ref: "#/components/schemas/destination-motherduck"
- title: destination-mssql
$ref: "#/components/schemas/destination-mssql"
- title: destination-mysql
$ref: "#/components/schemas/destination-mysql"
- title: destination-oracle
$ref: "#/components/schemas/destination-oracle"
- title: destination-pgvector
$ref: "#/components/schemas/destination-pgvector"
- title: destination-pinecone
$ref: "#/components/schemas/destination-pinecone"
- title: destination-postgres
$ref: "#/components/schemas/destination-postgres"
- title: destination-pubsub
$ref: "#/components/schemas/destination-pubsub"
- title: destination-qdrant
$ref: "#/components/schemas/destination-qdrant"
- title: destination-redis
$ref: "#/components/schemas/destination-redis"
- title: destination-redshift
$ref: "#/components/schemas/destination-redshift"
- title: destination-s3
$ref: "#/components/schemas/destination-s3"
- title: destination-s3-glue
$ref: "#/components/schemas/destination-s3-glue"
- title: destination-sftp-json
$ref: "#/components/schemas/destination-sftp-json"
- title: destination-snowflake
$ref: "#/components/schemas/destination-snowflake"
- title: destination-snowflake-cortex
$ref: "#/components/schemas/destination-snowflake-cortex"
- title: destination-teradata
$ref: "#/components/schemas/destination-teradata"
- title: destination-timeplus
$ref: "#/components/schemas/destination-timeplus"
- title: destination-typesense
$ref: "#/components/schemas/destination-typesense"
- title: destination-vectara
$ref: "#/components/schemas/destination-vectara"
- title: destination-weaviate
$ref: "#/components/schemas/destination-weaviate"
- title: destination-yellowbrick
$ref: "#/components/schemas/destination-yellowbrick"
```

## Proposed fix

To resolve, we should add this text to the `DestinationConfiguration` declaration in the OpenAPI spec:

```yml
DestinationConfiguration:
# ...
discriminator:
propertyName: destinationType
oneOf:
- title: destination-google-sheets
$ref: "#/components/schemas/destination-google-sheets"
# ...
```

and similarly for sources:

```yml
SourceConfiguration:
# ...
discriminator:
propertyName: sourceType
oneOf:
- title: ...
$ref: ...
# ...
```