Support elevated and extra elevated priorities in automatic fee priority algorithm
This issue requests supporting `elevated` or even extra elevated fee priority in wallet2's automatic fee priority algorithm, [`adjust_priority`](https://github.com/monero-project/monero/blob/257db6dff257bc9f60641b16d199ffde252655b2/src/wallet/wallet2.cpp#L8635).

Currently only `unimportant` and `normal` levels are supported, which might be insufficient during severe congestion.