hivesim-rs: allow test data to be generic

[morph-dev]

Problem

Currently, the hivesim-rs allows only specific type as a TestData. This is limiting in many ways:

• Many of the current simulations don't use any test data
• TestData only contains String objects, which require simulations to serialize/deserialize objects unnecessary
• Some simulations need exactly one content key-value pair, but they are using Vec because that's how it's defined
• When simulations need new specific type, they would first need to update hivesim-rs in one PR, in order to use that commit as dependency in their own PR.

Solution

Make test data generic. This removes all above problems without any downsides (in my opinion).
Example of how portal simulators would be updated can be seen here: 3cd8c95 (see #1151)

[KolbyML]