new: [Proposals] New get proposals index functions

[cvandeplas]

This PR adds the functionality to get the proposals list.
It also supports the filter for the timestamp that was introduced in MISP 2.4.111, but also works (without time-filter) on older MISPs.

[coveralls]

Coverage decreased (-2.3%) to 38.253% when pulling 6365333 on cvandeplas:master into 0198c87 on MISP:master.

[coveralls]

Coverage decreased (-0.04%) to 40.56% when pulling fa30d28 on cvandeplas:master into 0198c87 on MISP:master.

[Rafiot]

@cvandeplas Thanks!

@iglocska Is there a way to do that without the URL parameters?

[codecov]

Codecov Report

Merging #427 into master will decrease coverage by 0.03%.
The diff coverage is 12.5%.

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
- Coverage   40.59%   40.56%   -0.04%     
==========================================
  Files          34       34              
  Lines        5776     5784       +8     
==========================================
+ Hits         2345     2346       +1     
- Misses       3431     3438       +7

Impacted Files	Coverage Δ
pymisp/api.py	89.47% <ø> (ø)	⬆️
pymisp/aping.py	13.1% <12.5%> (-0.01%)	⬇️

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[codecov]

Codecov Report

Merging #427 into master will decrease coverage by 2.34%.
The diff coverage is 5.26%.

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
- Coverage   40.59%   38.25%   -2.35%     
==========================================
  Files          34       35       +1     
  Lines        5776     6146     +370     
==========================================
+ Hits         2345     2351       +6     
- Misses       3431     3795     +364

Impacted Files	Coverage Δ
pymisp/api.py	89.47% <ø> (ø)	⬆️
tests/testlive_comprehensive.py	0% <0%> (ø)	⬆️
tests/testlive_sync.py	0% <0%> (ø)
pymisp/abstract.py	86.88% <100%> (ø)	⬆️
pymisp/__init__.py	83.78% <100%> (ø)	⬆️
pymisp/mispevent.py	68.12% <57.14%> (-0.66%)	⬇️
pymisp/aping.py	12.71% <9.03%> (-0.4%)	⬇️
pymisp/tools/load_warninglists.py	33.33% <0%> (-13.34%)	⬇️

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[cvandeplas]

As discussed, another commit covers this functionality, except the proposal_index() function.
I'll raise another PR to include this function.

[Rafiot]

Right, ExpandedPyMISP.attribute_proposals almost does that, except the timestamp key.

The clean way here is probably to use kw_params in _prepare_request: it generates the parameters in URLs the cake php way.