[mbedtls] add support for version 3.0.0 (#7001)

This commit adds forward compatibility with mbedTLS 3.0.

Changes that affected OpenThread implementation:

- Rename mbedtls_*_ret() cryptography functions whose deprecated
  variants have been removed (migration guide)

- Replace MBEDTLS_SSL_MAX_CONTENT_LEN with MBEDTLS_SSL_IN_CONTENT_LEN
  and MBEDTLS_SSL_OUT_CONTENT_LEN options (migration guide)

- Signature functions now require the hash length to match the
  expected value (migration guide)

- SSL key export interface change (migration guide) - this change
  affected us a little bit more, since Key Block for KEK generation is
  not part of mbedTLS callback anymore. I was able, however, to
  retrieve it from Master secret (did successfully commissioning
  between 2.2x vs 3.0). By the way, I clean up the code for KEK
  generation - I don't know why we also executed it for application
  CoAP secure?

- Separated MBEDTLS_SHA224_C and MBEDTLS_SHA256_C (migration guide)

- Most structure fields are now private (migration guide) - we use
  private fields just in two files in dtls.cpp and ecdsa.cpp. For now,
  I used a workaround by defining MBEDTLS_PRIVATE macro but we should
  request the mbedTLS team to provide us support for public
  getters.

.github/workflows/build.yml

@@ -121,6 +121,25 @@ jobs:
       run: |
         script/check-scan-build
 
+  mbedtls3-build:
+    runs-on: ubuntu-18.04
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: true
+    - name: Bootstrap
+      run: |
+        sudo apt-get --no-install-recommends install -y ninja-build libreadline-dev libncurses-dev
+        rm -rf third_party/mbedtls/repo
+    - uses: actions/checkout@v2
+      with:
+        repository: ARMmbed/mbedtls
+        ref: v3.0.0
+        path: third_party/mbedtls/repo
+    - name: Build
+      run: |
+        ./script/test build
+
   arm-gcc:
     name: arm-gcc-${{ matrix.gcc_ver }}
     runs-on: ubuntu-18.04