Dynamically derive default mortality from System::Constants::BlockHashCount

[emostov]

BlockHashCount is defined by the runtime and thus we cannot know for sure what an ok mortal_period is. Ideally we could fetch the BlockHashCount from the runtime and do something like find the next greatest power of two and use that as the default mortal_period

Originally posted by @niklasad1 in #204 (comment)

[dvdplm]

Possibly related to #331. At the very least we should make it possible to specify the mortality.

[jsdw]

We can now specify the mortality. transactions default to immortal if you don't provide any params.

I could see an argument for transacitons defaulting to being mortal though, since it's a much safer default, so I guess this is still worth looking into!

[jsdw]

Since not all extrinsics have a mortality (it's an additional property that can vary by chain (and users can configure Subxt to have whatever additional things they want), I'm thinking to close this at this point. Users can specify Mortality at least using a slightly nice API when talking to Polkadot nodes :)