Add external directory for Conan recipe's exports

[legleux]

High Level Overview of Change

Adds a the external directory to the Conan exports required for generating the libxrpl Conan package.

Context of Change

Without exporting the contents of external/, package generation errors out with:

CMake Error at CMakeLists.txt:73 (add_subdirectory):
  add_subdirectory given source "external/secp256k1" which is not an existing
  directory.
CMake Error at CMakeLists.txt:74 (add_library):
  add_library cannot create ALIAS target "secp256k1::secp256k1" because
  target "secp256k1" does not already exist.
CMake Error at CMakeLists.txt:75 (add_subdirectory):
  add_subdirectory given source "external/ed25519-donna" which is not an
  existing directory.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.9%. Comparing base (7612805) to head (780f5c1).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5006     +/-   ##
=========================================
- Coverage     70.9%   70.9%   -0.0%     
=========================================
  Files          796     796             
  Lines        66792   66792             
  Branches     10996   10997      +1     
=========================================
- Hits         47381   47379      -2     
- Misses       19411   19413      +2     

see 2 files with indirect coverage changes

[thejohnfreeman]

There will be yet more new exports (include/, cmake/) after the physical redesign is complete, but this reminds me that I need to add them to the final PR (#4997).