Set more reasonable default header limits (64kb)

[mvanderlee]

What do these changes do?

Set more reasonable default header limits (64kb) and make them configurable via environment variables.
I opted to go with environment variables because it was a lot quicker and easier than traversing the entire callstack and adding it to the client code.

My use-case is to process responses from Trino, which has a default limit of 2MB, so configurability is a must.

Are there changes in behavior for the user?

No

Related issue number

#2304, at least partially.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[asvetlov]

NGINX has 8kb limit, aiohttp often follows nginx defaults.

Regarding dynamic limits configuration -- I agree, the feature is useful.
But the configuration should be controlled by Connector class, not globally by env vars.
aiohttp is a library, not a program.
If user wants to use env vars for configuration -- that is perfectly fine but out of aiohttp project scope.

I'm closing this PR but open for another proposal that configures HttpResponseParser in a way similar to timeouts, read_bufsize and auto_decompress configuration.