Add Bulla Signal adapter

[bengobeil]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Adds new Bulla Network signal adapter

• What is the current behavior? (You can also link to an open issue here)

A claim_id can be passed to the signal adapter to get signals for given receivable and parties

• What is the new behavior (if this is a feature change)?

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

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[height]

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[bengobeil]

What would be the best way to provide the subgraph_url setting param in the CI?