feat: add Degiro as a Feed and Broker

[fernandobrito]

And more small improvements:

• Standardize cache strategies on Brokers
• Add a thread lock on AvanzaBroker constructor
• Move Bar factory methods to Feeds
• Move some of my own constants (stop loss related) to a "strategy" file
• Add a few more fields to the stop loss endpoint
• Avoid caching HTTP requests to GCP APIs

Solves #10

[codecov[bot]]

Codecov Report

Merging #15 (a247faa) into main (441a617) will increase coverage by 0.40%. The diff coverage is 56.25%.

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   56.08%   56.48%   +0.40%     
==========================================
  Files          29       32       +3     
  Lines         863     1025     +162     
  Branches       68       83      +15     
==========================================
+ Hits          484      579      +95     
- Misses        371      435      +64     
- Partials        8       11       +3     

Impacted Files	Coverage Δ
src/investmentstk/models/price.py	100.00% <ø> (ø)
src/investmentstk/server.py	0.00% <0.00%> (ø)
src/investmentstk/formulas/average_true_range.py	70.73% <33.33%> (+6.14%)	:arrow_up:
src/investmentstk/data_feeds/degiro_feed.py	39.72% <39.72%> (ø)
src/investmentstk/brokers/degiro_broker.py	41.86% <41.86%> (ø)
src/investmentstk/models/stop_loss.py	87.50% <50.00%> (-12.50%)	:arrow_down:
src/investmentstk/models/asset.py	64.81% <60.00%> (-3.94%)	:arrow_down:
src/investmentstk/persistence/requests_cache.py	64.70% <66.66%> (+1.74%)	:arrow_up:
src/investmentstk/brokers/avanza_broker.py	50.00% <80.00%> (+6.66%)	:arrow_up:
src/investmentstk/utils/calendar.py	90.47% <81.81%> (+5.47%)	:arrow_up:
... and 12 more

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