update user agent to 2.1

[sbiscigl]

Description of changes:

Updates the user agent string to be compliant with the 2.1 revision. all SDKs will eventually update to this version. For example go v2 already has in this commit.

example before:

user-agent: 
  aws-sdk-cpp/1.11.479 
  ua/2.0 
  md/aws-crt#0.19.7 
  os/Darwin/23.6.0 
  md/arch#arm64 
  lang/c++#C++11 
  md/Clang#15.0.0 
  api/S3

example after:

user-agent: 
  aws-sdk-cpp/1.11.479 
  ua/2.1 
  api/S3 
  os/Darwin#23.6.0 
  lang/c++#C++11 
  md/aws-crt#0.19.7 
  md/arch#arm64 
  md/Clang#15.0.0 
  m/D

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• Linux
• Windows
• Android
• MacOS
• IOS
• Other Platforms

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[sbiscigl]

this was actually broken before too
os/Darwin/23.6.0 -> os/Darwin#23.6.0
which is actually the correct format and actually how CLI has it in their user agent string

aws-cli/2.16.5 
  md/awscrt#0.20.11 
  ua/2.0 
  os/linux#5.10.230-202.885.amzn2int.x86_64 
  md/arch#x86_64 
  lang/python#3.11.8 
  md/pyimpl#CPython 
  cfg/retry-mode#standard 
  md/installer#exe 
  md/distrib#amzn.2 
  md/prompt#off 
  md/command#ec2.describe-instances

[sbera87]

personally I would not use macros unless necessary
an alternative perhaps . Not a blocker just food for thought
#if defined(AWS_USER_AGENT_CUSTOMIZATION) constexpr const char* awsUserAgentCustomization = AWS_USER_AGENT_CUSTOMIZATION; #else constexpr const char* awsUserAgentCustomization = ""; #endif

and then ,
m_customizations{FilterUserAgentToken(awsUserAgentCustomization)}

[sbiscigl]

this is copied existing code, that we cannot remove for backwards compatible reasons.

[SergeyRyabinin]

constexpr const char* awsUserAgentCustomization = AWS_USER_AGENT_CUSTOMIZATION;

won't compile because, considering macro defined as -DmyCustomUserAgentToken, will be substituted by preprocessor to

constexpr const char* awsUserAgentCustomization = myCustomUserAgentToken; /* not a string literal! */

double macro is a standard approach here to convert macro value to a string literal.

[sbera87]

I was going by the existing definition which is quoted. Without that yes it will fail. Yes, usually we do double macro for cmdline flags . this was just food for thought for the internal ones that we set up /control as strings like this in cmake
set(AWS_USER_AGENT_CUSTOMIZATION "" CACHE STRING "User agent extension")

[sbera87]

Approving with comments on empty str checks , const correctness, copy constructibility . Functionally looks ok to me. Revisions look good