Support "custom implementations" of OAuth flows

[weberjm]

From @winklerj: while looking at Amazon's OAuth2 authentication https://github.com/amzn/selling-partner-api-docs/blob/main/guides/en-US/developer-guide/SellingPartnerApiDeveloperGuide.md#step-3-amazon-sends-you-the-authorization-information it looks like it varies slightly from what's currently available. To make it work there are some slight adjustments to be made configurable.

• At a minimum be able to configure the expected authorization code parameter. Right now it is hard coded to code. As an example Amazon uses spapi_oauth_code. Ideally you can configure the expected parameter name for all the required parameters, but default to their current parameter keys.
• Save additional parameters sent to the callback service. As an example shopify sends the shop, host and hmac. Amazon sends selling_partner_id and mws_auth_token.

Originally posted by @winklerj in #1213 (comment)

In the additional comments on this issue, it was noted that this does not conform to the OAuth standards, but Amazon and some other large providers have "custom implementations" that would be beneficial to support. This should be added to the OAuth support in the secret-service.

[weberjm]

Remainder of the comment flow from #1213:

• Enhance Secret Service Authorization Capabilities #1213 (comment)
• Enhance Secret Service Authorization Capabilities #1213 (comment)
• Enhance Secret Service Authorization Capabilities #1213 (comment)
• Enhance Secret Service Authorization Capabilities #1213 (comment)