feat(routing/http/server): improve ux of /ipns

[lidel]

• Content-Disposition with meaningful file extension
• default to application/vnd.ipfs.ipns-record when Accept is missing or includes */* wildcard.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 59.85%. Comparing base (b101ba0) to head (4c5650b).

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
+ Coverage   59.78%   59.85%   +0.06%     
==========================================
  Files         232      232              
  Lines       28124    28135      +11     
==========================================
+ Hits        16815    16839      +24     
+ Misses       9837     9822      -15     
- Partials     1472     1474       +2     

Files	Coverage Δ
routing/http/server/server.go	70.54% <84.61%> (+0.86%)	:arrow_up:

... and 15 files with indirect coverage changes