Address some issues around tx mortality

[jsdw]

• Make it a runtime error if you explicitly set an extrinsic to be .mortal(..) when providing params to it, but then are constructing it offline which prevents this from working. Previously we'd turn the extrinsic into an immortal one.

• Expose mortal_from_unchecked in the DefaultExtrinsicParamsBuilder to make it a little more ergonomic to set the relevant mortality params when constructing eg an offline extrinsic. Previously You'd have to do:

  let params = DefaultExtrinsicParamsBuilder::new();
  params.5 = CheckMortalityParams::mortal_from_unchecked(for_n_blocks, from_block_n, from_block_hash);

• Clarify the docs on these functions to make it clear that you can't use mortal() with offline extrinsics.

• Actually construct mortal transactions by default. I think this was the intended behaviour, but it may have been accidentally broken because the default Params actually led to immortal transactions.

• This led to uncovering an issue decoding Eras, and so now there is a proper implementation of EncodeAsType and DecodeAsType for it.

Closes #2024

[lexnv]

LGTM!