Feature Request: single environment variable config for protocol

Currently it is possible to provide overrides with environment variables of the style FSSPEC_{protocol}_{kwargname}=value. However, this is limited since it is not possible to provide a dict value. One example where this is needed is s3 with a custom endpoint_url, see https://github.com/fsspec/s3fs/issues/432.

I propose to support a single environment variable FSSPEC_{protocol} expecting its value to be json or yaml (parsed with pyyaml). It is just an extension so there would be no breaking changes. Current individual variables FSSPEC_{protocol}_{kwargname}=value would override the more general FSSPEC_{protocol}.

For example for the s3 protocol, the environment variable name would be FSSPEC_S3. Someone could set shell variables as:

$ FSSPEC_S3='
{
  "key": "my_key",
  "secret": "my_secret",
  "client_kwargs": {
    "endpoint_url": "https://custom-s3-host"
  }
}'
$ FSSPEC_S3_SECRET='my_other_secret'

Then the final arguments for the s3 protocol would be:

key = 'my_key'
secret = 'my_other_secret'
client_kwargs = {'endpoint_url': 'https://custom-s3-host'}

The only possible way to set endpoint_url would be through FSSPEC_S3, since for a more specific variable name such as FSSPEC_S3_CLIENT_KWARGS__ENDPOINT_URL there is no way to distinguish between an item child of a dict in client_kwargs or an argument whose name is client_kwargs__endpoint_url. This is not a limitation since endpoint_url without any other arguments can be set as:

$ FSSPEC_S3='
{
  "client_kwargs": {
    "endpoint_url": "https://custom-s3-host"
  }
}'

Locate the environment-variable override handling in the filesystem_spec codebase and inspect how protocol kwargs are currently parsed and merged. Implement the documented FSSPEC_{protocol} JSON or YAML input with specific variables taking precedence, then add or update tests covering nested values and override ordering.