fix(backport): handle custom param serializer fn

[JadeMugs]

Resolves issues #6274 and #6262.

Changes:

• Re-added the custom param serializer function and fully handled its functionality.
• Enabled the ability to pass both encode and serialize options in the paramsSerialize configuration parameter.

To ensure ease of maintenance, the implementation mirrors the one in v1.6.7 (latest version).

[briwa]

I've done something similar here: #6263 together with issue #6262. I'm not sure if we need another PR for this because I've opened mine 3 weeks ago and nobody has taken up on it yet.

[Xusifob]

Can this be merged ? I have a lot of "req is not defined" error and had to revert to 0.24.0

[briwa]

Can this be merged ? I have a lot of "req is not defined" error and had to revert to 0.24.0

@Xusifob A similar patch is merged in #6263 and has been released in v0.28.1. It should solve the same issue that this PR is addressing See: 3021e0d