Fix encode_client_secret_basic to match RFC 6749

[adamrimon]

Added url encoding of client_id and client_secret in encode_client_secret_basic per RFC 6749:
https://datatracker.ietf.org/doc/html/rfc6749#section-2.3.1

This fixes the unsupported situation where there is a colon character in the client_id or in the client_secret.

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What kind of change does this PR introduce? (check at least one)

• Bugfix - fixes Client ID containing a colon doesn't work with OAuth2's client_secret_basic authentication method #595
• Feature
• Code style update
• Refactor
• Other, please describe:

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[lepture]

@Prilkop reverted, it caused too many bugs.

You can quote your client_id and client_secret at first in your own code.

[adamrimon]

I agree with that I can quote the client_id and client_secret in my own code, but that's clearly against the RFC (requiring both to be url-encoded).
The bugs I saw were of people using previous version along with the new one - of course that could lead to a bug as this change fixes the previous incompatible wrong behavior (that is incompatible with other, compliant, servers).