feat: async melt requests

[cloudsupper]

this PR adds support for Prefer: respond-async header on the POST v1/melt endpoint. If a request contains such header, the Mint no longer blocks until the operation is completed, but rather it schedules a task for the backend and return immediately with a PENDING melt quote response.

Relevant specification PR is here

[codecov[bot]]

Codecov Report

:x: Patch coverage is 84.12698% with 10 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 64.21%. Comparing base (a5f950a) to head (572ece8). :warning: Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
cashu/mint/ledger.py	85.96%	8 Missing :warning:
cashu/mint/router.py	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #801      +/-   ##
==========================================
+ Coverage   61.12%   64.21%   +3.08%     
==========================================
  Files          89       90       +1     
  Lines       10565    10875     +310     
==========================================
+ Hits         6458     6983     +525     
+ Misses       4107     3892     -215     

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