Set default pjsip_cred_info.auth_algorithm to PJSIP_AUTH_ALGORITHM_MD5

[gtjoseph]

The new algorithm_type field in the pjsip_cred_info structure wasn't being
defaulted to PJSIP_AUTH_ALGORITHM_MD5 as it should be. As a result if the
user specified a data_type of PJSIP_CRED_DATA_DIGEST but didn't explicitly
set algorithm_type, it was left at PJSIP_AUTH_ALGORITHM_NOT_SET which
will caused authentication to fail.

• pjsip_cred_info.auth_algorithm is now correctly defaulted to
  PJSIP_AUTH_ALGORITHM_MD5 when supplied via the pjsip_auth_lookup_cred and
  pjsip_auth_lookup_cred2 callbacks and via the pjsip_auth_clt_set_credentials,
  and pjsip_auth_create_digest functions.

• pjsip_cred_info.auth_algorithm now defaults to PJSIP_AUTH_ALGORITHM_SHA256
  for the pjsip_auth_create_digestSHA256 function.

• Documentation was updated for those functions and callbacks to indicate the
  defaults.

NOTE: The pjsip_auth_create_digest and pjsip_auth_create_digestSHA256
defaults are actually set in pjsip_auth_create_digest2 because those two
functions are now deprecated wrappers that pass cred_info (which is a const) to
pjsip_auth_create_digest2. The documentation for pjsip_auth_create_digest2
however, doesn't mention defaults on purpose because it's a generic function
that handles multiple algorithms and we want users to specifify exactly what
algorithms they want to use to avoid ambiguity.

Resolves: #4194

[nanangizz]

This behavior seems to be specified in pjsip_auth_create_digest2() declaration:

pjproject/pjsip/include/pjsip/sip_auth.h

Lines 797 to 798 in 8117bc5

	* \note If left unset (0), pjsip_cred_info::algorithm_type will
	* default to #PJSIP_AUTH_ALGORITHM_MD5.

)
while the 'defaulting' action seems to be done in pjsip_auth_clt_set_credentials(), so I think the specification needs to be moved to pjsip_auth_clt_set_credentials()?
Also, perhaps the spec should mention about "when data_type is set to PJSIP_CRED_DATA_DIGEST".

[gtjoseph]

@nanangizz I updated the documentation, PR description and commit message. Since the pjsip_auth_create_digest* functions are public, I also updated them. See the "NOTE" in the PR description and see if that makes more sense.

[nanangizz]

... and we want users to specifify exactly what
algorithms they want to use to avoid ambiguity

Agree. Honestly I was wondering why not just enforce it, e.g: pjsip_auth_clt_set_credentials() returns error when pjsip_cred_info::algorithm_type is not set (or even when it is set but not supported).

[gtjoseph]

... and we want users to specifify exactly what
algorithms they want to use to avoid ambiguity

Agree. Honestly I was wondering why not just enforce it, e.g: pjsip_auth_clt_set_credentials() returns error when pjsip_cred_info::algorithm_type is not set (or even when it is set but not supported).

I didn't want to break backwards compatibility (any more than I already did). :)