Use private variable instead of immutable to allow calls to _msgSender in constructor.

[Amxx]

Immutable _trustedForwarder causes issues when _msgSender() is called in constructors

Fixes #2739

PR Checklist

• Tests
• Changelog entry

[drortirosh]

Sorry, but I think this solution is sub-optimal, compared to the solution in https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2739/files

• the _trustedForwarder is a member modified only by the constructor. there is no need to make it private (which also costs more.)
• the issue is not with the ERC2771Context at all (which indeed, works with any other use-case, e.g. with ERC20, AccessControl ) but with the Ownable contract: _msgSender() is completely meaningless in the constructor code, as it can't be called directly via a meta-transaction call.

[frangio]

@drortirosh Thanks for the feedback. The reasons why we went with this change instead are:

1. The problem extends beyond Ownable. Contracts like AccessControl are also affected.
2. In some of those cases, _msgSender is not used directly in the constructor but indirectly through an internal function, and this becomes a lot harder to work around.
3. Other variants of Context._msgSender could technically make sense in the constructor, so we prefer to continue using it consistently.

Additionally, the Solidity team has hinted that this limitation could soon be lifted at the compiler level, and we plan to add immutable back then. ethereum/solidity#10463 (comment)