Consider providing an alternative way to configure SNI via configuration provider

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

Docs: _[SNI in configuration](https://learn.microsoft.com/en-us/aspnet/core/fundamentals/servers/kestrel/endpoints#sni-in-configuration)_

Related issue: https://github.com/dotnet/aspnetcore/issues/15144

---
Most Unix shells do not support special characters such as dot (.), hyphen (-), and asterisk (*) in environment variable names:
```bash
# bash --version
GNU bash, version 5.2.15(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

```
```bash
# export DOMAIN_NAME=localhost
# echo $DOMAIN_NAME
localhost

# export DOMAIN.NAME.WITH.DOTS=my.domain.name
-bash: export: `DOMAIN.NAME.WITH.DOTS=my.domain.name': not a valid identifier

# export DOMAIN-NAME-WITH-HYPHENS=my-domain-name
-bash: export: `DOMAIN-NAME-WITH-HYPHENS=my-domain-name': not a valid identifier

# export DOMAIN_NAME_PATTERN_*=*.example.com
-bash: export: `DOMAIN_NAME_PATTERN_*=*.example.com': not a valid identifier
```

Current SNI configuration uses domain names as keys, so this prevents it from being configured using environment variables:
```bash
# export KESTREL__ENDPOINTS__HTTPS__URL="https://*"
# echo $KESTREL__ENDPOINTS__HTTPS__URL
https://*

# export KESTREL__ENDPOINTS__HTTPS__SNI__localhost__CERTIFICATE__SUBJECT=""
# echo $KESTREL__ENDPOINTS__HTTPS__SNI__localhost__CERTIFICATE__SUBJECT

# export KESTREL__ENDPOINTS__HTTPS__SNI__*.example-domain.com__CERTIFICATE__SUBJECT=""
-bash: export: `KESTREL__ENDPOINTS__HTTPS__SNI__*.example-domain.com__CERTIFICATE__SUBJECT=': not a valid identifier
```
There is a workaround for this:
```bash
# env "KESTREL__ENDPOINTS__HTTPS__SNI__*.example-domain.com__CERTIFICATE__SUBJECT=" bash
# perl -e 'print $ENV{"KESTREL__ENDPOINTS__HTTPS__SNI__*.example-domain.com__CERTIFICATE__SUBJECT"}'

```
But we still have (and will have) compatibility issues in various scenarios. For example, this doesn't work in docker compose:
```ini
# .env

SNI_DOMAIN="*.example-domain.com"
```
```yaml
# docker-compose.yml

services:
host:
environment:
- KESTREL__ENDPOINTS__HTTPS__SNI__${SNI_DOMAIN}__CERTIFICATE__SUBJECT=""
```

### Describe the solution you'd like

The Endpoints section consists of custom endpoint names with the Url key inside:
```json
{
"Kestrel": {
"Endpoints": {
"HTTP": {
"Url": "http://*:80"
},
"HTTPS": {
"Url": "https://*:443"
}
}
}
}
```
It would be nice to use a similar approach in the Sni section (here I use the Domain key):
```json
{
"Kestrel": {
"Endpoints": {
"HTTP": {
"Url": "http://*:80"
},
"HTTPS": {
"Url": "https://*:443",
"Sni": {
"LocalHost": {
"Domain": "localhost",
"Certificate": {
"Subject": "",
"Path": "",
"Password": "$CREDENTIAL_PLACEHOLDER$"
}
},
"ExampleWithSubdomains": {
"Domain": "*.example-domain.com",
"Certificate": {
"Subject": "",
"Path": "",
"Password": "$CREDENTIAL_PLACEHOLDER$"
}
},
"Fallback": {
"Domain": "*"
}
},
"Certificate": {
"Path": "",
"Password": "$CREDENTIAL_PLACEHOLDER$"
}
}
},
"Certificates": {
"Default": {
"Path": "",
"Password": "$CREDENTIAL_PLACEHOLDER$"
}
}
}
}
```

### Additional context

Perhaps useful additions to the configuration-related decision checklist:
- generally, configuration keys should be treated as C_IDENTIFIER (with some exceptions, such as logging categories);
- configuration values should not be used as keys.

This approach will ensure maximum possible compatibility between different platforms.