Add patched recipe for SOCI: (#4510)

SOCI is the C++ database access library. The SOCI recipe was updated in
Conan Center Index (CCI), and it breaks for our choice of options. This
breakage occurs when you build with a fresh Conan cache (e.g. when you
submit a PR, or delete `~/.conan/data`).

* Add a custom Conan recipe for SOCI v4.0.3
* Update dependency building to handle exporting and installing Snappy
  and SOCI
  * Fix workflows to use custom SOCI recipe
* Update BUILD.md to include instruction for exporting the SOCI Conan
  recipe:
  * `conan export external/soci soci/4.0.3@`

This solution has been verified on Ubuntu 20.04 and macOS.

Context:

* There is a compiler error that the `sqlite3.h` header is not available
  when building soci.
* When package B depends on package A, it finds the pieces it needs by
  importing the Package Configuration File (PCF) that Conan generates
  for package A.
  * Read the CMake written by package B to check that it is importing
    the PCF correctly and linking its exports correctly.
  * Since this can be difficult, it is often more efficient to check
    https://github.com/conan-io/conan-center-index/issues for package B
    to see if anyone else has seen a similar problem.
  * One of the issues points to a problem area in soci's CMake. To
    confirm the diagnosis, review soci's CMake (after any patches are
    applied) in the Conan build directory `build/$buildId/src/`.
  * Review the Conan-generated PCF in
    `build/$buildId/build/$buildType/generators/`.
  * In this case, the problem was likely (re)introduced by
    https://github.com/conan-io/conan-center-index/pull/17026
* If there is a problem in the source or in the Conan recipe, the
  fastest fix is to copy the recipe and either:
  * Add a source patch to fix any problems in the source.
  * Change the recipe to fix any problems in the recipe.
* In this case, this can be done by finding soci's Conan recipe at
  https://github.com/conan-io/conan-center-index/tree/master/recipes/soci
  and then copying the `all` directory as `external/$packageName` in our
  project. Then, make any changes.
  * Test packages can be removed from the recipe folder as they are not
    needed.
  * If adding a patch in the `patches` directory, add a description for
    it to `conandata.yml`.
  * Since `conanfile.py` has no `version` property on the recipe class,
    builders need to pass a version on the command line (like they do
    for our `snappy` recipe).
* Add an example command to `BUILD.md`.

Future work: It may make sense to refer to recipes by revision, by
checking in a lockfile.

BUILD.md

@@ -127,6 +127,13 @@ which allows you to statically link it with GCC, if you want.
 conan export external/snappy snappy/1.1.9@
 ```
 
+Export our [Conan recipe for SOCI](./external/soci).
+It patches their CMake to correctly import its dependencies.
+
+```
+conan export external/soci soci/4.0.3@
+```
+
 ## How to build and test
 
 Let's start with a couple of examples of common workflows.