Adding MLFlow new methods, Julia LTS checks and tests

[pebeto]

This PR contains:

• logbatch function to upload params, metrics and tags in a single call.
• restoreexperiment to restore a deleted experiment.
• MLFlowRunDataParam to standardize the way we retrieve information from the API.
• Adding Julia LTS (1.6) inside CI.
• Adding tests (specially one using IOBuffer inside logartifact).

[codecov-commenter]

Codecov Report

Merging #28 (074c9a0) into main (6e13f32) will decrease coverage by 0.80%. The diff coverage is 86.20%.

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@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
- Coverage   79.86%   79.07%   -0.80%     
==========================================
  Files           9        9              
  Lines         303      325      +22     
==========================================
+ Hits          242      257      +15     
- Misses         61       68       +7     

Impacted Files	Coverage Δ
src/MLFlowClient.jl	100.00% <ø> (ø)
src/experiments.jl	82.85% <70.00%> (-5.67%)	:arrow_down:
src/types/runs.jl	82.85% <91.66%> (+0.04%)	:arrow_up:
src/loggers.jl	69.87% <100.00%> (+2.77%)	:arrow_up:

... and 1 file with indirect coverage changes

[deyandyankov]

looking good @pebeto - do you need me to release a new version?