Add support for EPSG:27700

[TimG1964]

ERROR: LoadError: ArgumentError: EPSG/ESRI code for the ESRI ID "British_National_Grid" not found in dictionary.
Please check https://github.com/JuliaEarth/CoordRefSystems.jl/blob/main/src/strings.jl
If you know the EPSG/ESRI code of a given ESRI WKT string, please submit a pull request.

I'm reading this shape file from here at the the UK Office for National Statistics (ONS): LAD_DEC_2023_UK_BGC.shp.
Google tells me the British National Grid is EPSG:27700.

[juliohm]

We added the code and corresponding type as discussed in https://discourse.julialang.org/t/esri-code-for-british-national-grid-not-known-by-geoio/117641

I will leave this issue open because we still need to work on a final missing feature to make conversions 100% correct.