Add proj:code to support Projection extension v2

[jsignell]

When stac-extensions/projection#12 lands in the projection extension and a 2.0 is released, proj:epsg will be replaced by proj:code. I am not sure how this libaray handles different versions of extensions.

I was working on stac-utils/pystac#1287 and noticed that pystac depends on this library for summary fields, so just wanted to open this.

[m-mohr]

Implemented in ebd9c99