Change name of "extent_size" in Crucible Agent API

[smklein]

According to @leftwo 's comment: oxidecomputer/omicron#511 (comment)

In Crucible, extent_size is the number of block_size parts that make up a single extent. (It is not a good name, agreed).

I misinterpreted the name "extent_size" to mean "size of an extent, in bytes".

Could we change the name to reflect that it is a count, not a size?

I'd suggest something like: blocks_per_extent

[leftwo]

Oh @jclulow Namer of names. Do you have any compliant if I change extent_size to blocks_per_extent. I believe you
authored the original name.

Speak now, or forever hold your peace!