Fine-grained hot reloading configuration for functions and layers to replace global `mountCode` option

还没有人认领这个 Issue。

The current hot reloading configuration `mountCode` has many limitations (e.g., global config, no layer support). An improved configuration API and documentation could make hot reloading more powerful and easier to use with the Serverless Framework.

## Related Issues/PRs

* https://github.com/localstack/serverless-localstack/pull/222
* https://github.com/localstack/serverless-localstack/issues/129
* https://github.com/localstack/serverless-localstack/issues/216

## Challenges

* `mountCode: true` is a global option but currently does not support hot-reloading of layers (new feature in LocalStack 2.0).
* Hot reloading (currently) only works with at most one layer per function. Hence, if we intend to apply mountCode to layers as well, it fails if there is any function with more than one layer in the serverless.yml
* `mountCode` is bad terminology because the current implementation does not do any mounting anymore but rather copies ZIP files (https://docs.localstack.cloud/user-guide/tools/lambda-tools/hot-reloading/). `hotReload` would be a better name
* We need to consider the scope of hot reloading because not everyone wants to enable hot reloading globally. Selectively enabling hot reloading for single functions or layers under development would be helpful.
* `mountCode: true` uses the current directory (assumed to be project directory) as default hot reload path or accepts relative paths (e.g., ./functions). This might lead to many unnecessary and slow reloads.

## Background

* Serverless documentation on how to provide custom plugin configuration options: https://www.serverless.com/framework/docs/guides/plugins/custom-configuration
* It is possible to add custom function-level properties using "Function properties via defineFunctionProperties" via defineFunctionProperties
* There is no well-defined mechanism to add layer-level properties. It might be possible to do manually using `addPropertiesToSchema` to add something like `defineLayerProperties`?! Might need to relax provider schema by allowing additional properties like for functions.patternProperties here https://github.com/serverless/serverless/blob/ed15cb27aee68954c93d875da96274914943ad71/lib/classes/config-schema-handler/index.js#L335
* There is some API to extend configuration: https://www.serverless.com/framework/docs/guides/plugins/extending-configuration
* Serverless v3 comes with breaking changes regarding plugin variable resolving (e.g., comes with stricter validation): https://www.serverless.com/framework/docs/guides/upgrading-v3#new-variable-resolver-engine

## User Stories

Enable hot reloading for:
1. Single function code
2. Single function code + single layer
3. Single layer
4. Two functions sharing the same hot-reloaded layer

These cases are supported by LocalStack but not by the serverless-localstack plugin.

## Possible API

```yml
service: myService

plugins:
- serverless-localstack

custom:
localstack:
# DEPRECATED global config
# lambda:
# mountCode: true
stages:
- local
host: http://127.0.0.1

provider:
name: aws
region: us-east-1
runtime: python3.9
# OPTIONAL provider-scoped config here (would need to think about function vs. layers scope)

# https://www.serverless.com/framework/docs/providers/aws/guide/layers
layers:
layerOne:
path: layerOne
hotReload: true

# https://www.serverless.com/framework/docs/providers/aws/guide/functions
functions:
hello:
handler: handler.handler
hotReload: true
layers:
- !Ref LayerOneLambdaLayer
```

### Considerations

1. Hot reloading for layers is configured per layer (rather than per layer usage) and applies to all functions using the layer
2. LocalStack (currently) support at most one layer in total when using hot reloading for a layer. This makes it difficult to apply hot reloading globally for layers.
3. Should we consider supporting ZIP files for hot reloading such that we can re-use build steps from tools such as Serverless, Gradle, etc rather than adding extra steps to unzip these archives ([Java Gradle example](https://docs.localstack.cloud/user-guide/tools/lambda-tools/hot-reloading/#hot-reloading-for-jvm-lambdas)).

### Alternatives to create layers with Serverless

Based on https://www.tutorialspoint.com/serverless/serverless_layer_creation.htm

1. Creating python-requirements layer
4. Using an existing layer from another function in the same region
5. Non-requirements/ generic layers (example: https://www.serverless.com/blog/publish-aws-lambda-layers-serverless-framework)

首先查看相关的 PR #222 和现有的 custom.localstack.lambda.mountCode 配置，然后将提议的函数级和层级 hotReload 属性与链接的 Serverless 配置及 schema-extension 文档进行比较。四个列出的用户故事均受支持、层行为已定义，并且已记录弃用的全局选项的替代方案，即视为完成。

由索引模型根据 Issue 内容生成。