proposal: x/oauth2: support Refresh Token Request with Custom Parameters

[andig]

Migrated from golang/oauth2#521, refs #56402 (comment)

We'd like to be able to add custom parameters to the Refresh Token Request provided by the configuration client.

Proposed solution:

Add optional parameters to oauth2.Config.TokenSource() similar to SetAuthURLParam. This pattern could later be extended for headers if necessary.

Proposed API:

type TokenSourceOption func(*reuseTokenSource)
func (c *Config) TokenSource(ctx context.Context, t *Token, opts ...TokenSourceOption) TokenSource

Would be happy to try a CL.

Consequence of not implementing:

Custom unmarshaling of oauth2.Token (see #61417) or custom token refreshers (see https://github.com/evcc-io/evcc/blob/master/util/oauth/tokensource.go#L12)

Alternatives:

To some extend #61417 but still at cost of additional code.