feat(transaction-builder): Add support for extensible async signer

[DaughterOfMars]

Description

Adds a new trait to the transaction builder crate, Signer, which defines one simple method sign to sign a transaction asynchronously. This is implemented for any type that implements IotaSigner and thus supports basic signing as well as passing the transaction to other services which hold the appropriate secrets.

Closes #436

[coveralls]

Pull Request Test Coverage Report for Build 19886963795

Details

• 14 of 28 (50.0%) changed or added relevant lines in 5 files are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.007%) to 85.047%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/iota-sdk-transaction-builder/src/error.rs	0	3	0.0%
crates/iota-sdk-transaction-builder/src/builder/mod.rs	2	7	28.57%
crates/iota-sdk-transaction-builder/src/builder/gas_station.rs	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
crates/iota-sdk-transaction-builder/src/builder/gas_station.rs	1	0.0%
crates/iota-sdk-types/src/crypto/signature.rs	1	74.78%
crates/iota-sdk-crypto/src/simple.rs	7	75.0%

Totals
Change from base Build 19879363869:	-0.007%
Covered Lines:	20965
Relevant Lines:	24651

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💛 - Coveralls

[thibault-martinez]

What does this PR mean for the FFIs?

[DaughterOfMars]

What does this PR mean for the FFIs?

No change to that API

[thibault-martinez]

What does this PR mean for the FFIs?

No change to that API

I know but my point was that maybe it should? Bindings won't have traits so the existing methods still take a SimpleKeyPair, what will happen when we want another method for a new signer? We should at least rename the existing ones to have key_pair in the name?