Fix msvc warnings

[weberph2]

This fixes build errors/warnings that occur when building --with-tpm2 on Windows.

Environment: Visual Studio 2022 17.12.3, MSVC 14.42.34433, x64, tpm2-tss 4.1.3, nmake & ninja

e.g.: python.exe configure.py --enable-shared --cc=msvc --cpu=x64 --disable-deprecated-features --werror-mode --module-policy=bsi --debug-mode --library-suffix=D --program-suffix=D.exe --msvc-runtime=MDd --with-build-dir=build_debug_symbol --enable-modules=tpm2,tpm2_rsa,tpm2_ecc,tpm2_crypto_backend --with-tpm2 --with-external-includedir=c:/path/to/tpm2-tss/include --with-external-libdir=c:/path/to/tpm2-tss/x64/Debug --define-build-macro NOMINMAX --build-tool=ninja

The following warnings/errors are fixed:

1. use BOTAN_DIAGNOSTIC_IGNORE_INHERITED_VIA_DOMINANCE (warning C4250) for Botan::TPM2::EC_PublicKey and Botan::TPM2::RSA_PublicKey

build_debug_symbol\build\include\public\botan/tpm2_rsa.h(15): error C2220: the following warning is treated as an error
build_debug_symbol\build\include\public\botan/tpm2_rsa.h(15): warning C4250: 'Botan::TPM2::RSA_PublicKey': inherits 'Botan::RSA_PublicKey::Botan::RSA_PublicKey::algo_name' via dominance

build_debug_symbol\build\include\public\botan/ecc_key.h(99): note: see declaration of 'Botan::EC_PublicKey::estimated_strength'
build_debug_symbol\build\include\public\botan/tpm2_ecc.h(16): warning C4250: 'Botan::TPM2::EC_PublicKey': inherits 'Botan::EC_PublicKey::Botan::EC_PublicKey::get_int_field' via dominance

2. move defaulted move-constructor/-assignment operator from header to source file to fix "deletion of pointer to incomplete type 'Botan::TPM2::Context::Impl'"

error C2027: use of undefined type 'Botan::TPM2::Context::Impl'
botan/tpm2_context.h(155): note: see reference to class template instantiation 'std::unique_ptr<Botan::TPM2::Context::Impl,std::default_delete<Botan::TPM2::Context::Impl>>' being compiled

error C2338: static_assert failed: 'can't delete an incomplete type'
botan/tpm2_context.h(154): note: see declaration of 'Botan::TPM2::Context::Impl'

3. fix warning C4267: conversion from 'size_t' to '...', possible loss of data

botan/internal/tpm2_util.h(145): warning C4267: '=': conversion from 'size_t' to 'UINT16', possible loss of data
src/lib/prov/tpm2/tpm2_context.cpp(200): warning C4267: '-=': conversion from 'size_t' to 'uint32_t', possible loss of data
src/lib/prov/tpm2/tpm2_rng.cpp(41): warning C4267: 'argument': conversion from 'size_t' to 'UINT16', possible loss of data
src/lib/prov/tpm2/tpm2_key.cpp(125): warning C4267: '=': conversion from 'size_t' to 'UINT16', possible loss of data

4. remove duplicate return statement (fixes warning C4702: unreachable code)

src\lib\prov\tpm2\tpm2_hash.cpp(50) : warning C4702: unreachable code

/cc @FAlbertDev @reneme @atreiber94

[coveralls]

coverage: 91.262% (+0.003%) from 91.259%
when pulling b307ce8 on weberph2:fix-msvc-warnings
into 6943449 on randombit:master.

[randombit]

Beyond the open discussion issue, lgtm.

[FAlbertDev]

Changes since my last review look good to me 😉