Fix encode_client_secret_basic to match RFC 6749

[Prilkop]

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)

• [x] Bugfix - fixes #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.