Add 'server' field to MongoDB metadata to support DNS SRV in connection URI

[dmitsh]

Description

Added 'server' field to MongoDB metadata to support DNS seedlist connection format.
For non-empty server field, the connection URI is set to 'mongodb+srv:///'

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #904

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation / Created issue in the https://github.com/dapr/docs/ repo: Update documentation about MongoDB metadata docs#1568

[berndverst]

LGTM

[codecov]

Codecov Report

Merging #947 (6f521ab) into master (d1b898b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #947      +/-   ##
==========================================
+ Coverage   35.45%   35.47%   +0.02%     
==========================================
  Files         135      135              
  Lines       10642    10646       +4     
==========================================
+ Hits         3773     3777       +4     
  Misses       6487     6487              
  Partials      382      382              

Impacted Files	Coverage Δ
state/mongodb/mongodb.go	15.97% <100.00%> (+2.03%)	⬆️

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