Replace explorer prefix search with exact match for block and transaction hash

[imabdulbasit]

addresses https://github.com/EspressoSystems/espresso-block-explorer/issues/148

This PR:

• replaces the prefix search with equality for block and transaction searches
• replace search query parameter type from String to TaggedBase64
• queries database for blocks or transactions based on the tag

[Ayiga]

@Ayiga can you confirm this will work with the block explorer?

I think this should be fine in terms of functionality at the moment. There is a slight change in the input requirements that may cause problems with the current version of the Block Explorer. That being the requirement of the input to be a TaggedBase64. Though I do think that this is a good change, it does mean that the unchanged search request will start returning 4xx errors where it before didn't.

But this is likely alright, as we'll want to change the Block Explorer's search behavior regardless so that it isn't submitting these requests when they don't conform to the TaggedBase64 requirements.

[Ayiga]

Just tested this. The 4xx errors aren't affecting the current block explorer in any meaningful way. So this should be relatively safe. Though, gain we'd want to update the logic in the block explorer to not send partial TaggedBase64 search terms.