fix: notify commands that are >4096 bytes

[JeremyTubongbanua]

- What I did notify fixes & refactors

• Fixed a bug where notify commands that were greater than 4096 bytes would not execute. This is because the cmdbuffer was a constant size of 4096, instead of dynamically evaluating how much memory to allocate for the cmd string. This PR fixes that.
• applied base64 buffer optimizations
• New helper functions (for atclient_notify function)
• New functional test on atclient_notify for a long notiifcation (8012 bytes)

static int generate_cmd(const atclient_notify_params *params, const char *cmdvalue, const size_t cmdvaluelen,
                        char **ocmd, size_t *ocmdolen);
static size_t calculate_cmd_size(const atclient_notify_params *params, const size_t cmdvaluelen, size_t *atkeyolen,
                                 unsigned long long *ottln, size_t *medatastrolen);

NEW base64 stuff

• New base64 functions (for calculating optimal base64 length)

  size_t atchops_base64_encoded_size(const size_t plaintextsize);
  size_t atchops_base64_decoded_size(const size_t encodedsize);

• base64 tests on various encode/decode lengths

Chore stuff:

• Removed sudo from at_talk script

- How I did it

- How to verify it

• Added test for a VERY long notification
• Added base64 tests to test optimal buffer length functions
• Manual test at_talk as of this commit

- Description for the changelog fix: notify command >4096 bytes