Provide helper methods for Transformer plugins

[thilp]

Follow-up after TIP/docs#327 “Extract helper methods from existing Transformer plugins”.

We could go with the following types of helper methods:

• injecting (header, body parameter, query parameter): adding new value to existing ones, either to all requests or only to requests matching some filter;
• removing (header, body parameter, ...): removing value either from all requests or only the ones matching some filter;
• modifying (headers, body, url, ...): removing old value and using new one, either to all requests or only to requests matching some filter;
• removing the request based on filter (plugin based blacklist?).

ℹ️ Imported from TIP/docs#336.